This invention relates generally to optical fibers and more specifically to low-loss optical fibers having a fluoride glass cladding.
Optical waveguides have been known and used for sometime. As noted by M. D. Rigterink, in "Material Systems, Fabrication and Characteristics of Glass Fiber Optical Waveguides", Ceramic Bulletin, Vol. 55, No. 9 (1976), incorporated herein by reference, three basic types of optical waveguides are employed in communications. Multimode, stepped index profile waveguides are used for communications over short distances. As the term multimode implies, these waveguides are generally used in conjunction with an incoherent light source, such as an LED. Considerable pulse broadening occurs because of the variation in path lengths in the core (with constant refractive index) and thus of the arrival times of the various modes at the output end of the fiber.
The single mode type, which supports propagation of only one mode due to the small diameter of the core, is intended for very long distance (many kilometers between repeaters), high capacity communication systems. A solid state laser is usually the light source used with these fibers since it is the only light source capable of launching sufficient power into the single propagating mode. A favorable structure consists of a small diameter core surrounded by a cladding having a slightly smaller refractive index, in turn surrounded by a supporting jacket. The latter serves to increase the strength and chemical durability of the fiber but does not serve any optical role. The respective diameters are typically about 10, 100 and 150 microns. The difference in refractive index between core and cladding are relatively small (.about.0.2%) compared to other optical fibers. The numerical aperture is thus also small.
For intermediate distance applications, it is preferable to use multimode fibers with a graded index profile. Pulse broadening, resulting from variation in path length of the various modes of light propagating down the waveguide, is reduced as compared to the stepped index multimode fiber. With the graded index profile, most of the light rays travel as helical waves through regions of lower refractive index than exist at the center of the core. The velocity is greater in the regions of lower refractive index so that the arrival times for the various modes are more nearly the same. This results in greater bandwidth capability as the signals can travel for longer distances before degradation occurs because of different mode velocities. Ideally in single mode fibers, the only pulse broadening that occurs is primarily caused by material dispersion (variation of refractive index with wavelength). With the graded index fiber, the core and cladding diameters are typically about 50 and 100 microns and the refractive index difference, .about.1%.
In addition to pulse-broadening, transmission loss also limits the distance an optical waveguide can carry light. Transmission loss occurs because of several factors. Impurities in the waveguides absorb some of the transmitted light. In addition, thermal compositional fluctuations, phase separations, inhomogeneities within the waveguide as well as geometric variations in the size of the fiber core scatter a portion of the transmitted light. If splices must be made because sufficiently long waveguides may not be produced from availibe preforms, these splices further increase transmission loss.
Because fluoride glasses are several orders of mangitude more transparent than conventional silica based glass, fluoride glass has been often mentioned as a material from which to make efficient, low-loss optical fibers. Until now, however, several difficulties has made the use of fluoride glass in optical fibers impractical. Conventional cladding techniques, such as a chemical vapor deposition, cannot be used to make a fluoride glass preform because of the high vapor pressures of fluoride raw materials.
Further, to date, optical fibers having a fluoride glass cladding could only be prepared by the process of Mitachi and Miyashita, described in "Preparation of Low-Loss Fluoride Glass Fibers" Electron. Lett., Vol. 18, pp. 170-171 (1982), incorporated herein by reference. According to that process the fluoride cladding melt is poured into a mold which is then upset. The center of the melt flows out and a cylindrical tube is thus formed. Next, the fluoride core melt is poured in to form a preform. The limitations and disadvantages of this process are as follows:
(a) Due to the rapid change in the fluoride glass viscosity with respect to temperature, the cylindrical tube obtained by upseting the mold is not concentric which leads to undesirable variations in the preform core-clad ratio.
(b) Again due to this high viscosity dependence on temperature, the preparation of long preforms, and therefore long waveguides, is not possible.
(c) And finally, this process appears to be limited to step-index multimode fluoride fibers.